replace validation layer with VK_LAYER_KHRONOS_validation

[jdtournier]

This replaces the use of the VK_LAYER_LUNARG_standard_validation layer with the standard VK_LAYER_KHRONOS_validation version, making it possible to run with a standard setup (without the LunarG SDK).

I'm not sure whether there are other reasons to prefer the LunarG validation layer - this is very much my first look at Vulkan, I'm just very grateful to have access to this demo!